This invention relates to below-the-strain-point ion exchange strengthening techniques as taught by Weber in U.S. Pat. No. 3,218,220. In particular, the invention deals with strengthening soda-lime-silica glass compositions containing sodium, whereby the sodium ions in the glass are replaced by larger potassium ions from an external source to induce compression at the surface of the glass. Because of its substantial sodium oxide content conventional soda-lime-silica glass of the type found in flat glass may be used for ion exchange strengthening, but the treatment time required to attain adequate strength and compression layer depth is often longer than desired. Many attempts have been made to produce glass compositions which can be ion exchange strengthened more rapidly, chief among which are the Al.sub.2 O.sub.3 and/or ZrO.sub.2 containing glasses disclosed by Mochel in U.S. Pat. Nos. 3,485,702, 3,752,729, and 3,790,430. Such specially adapted glass compositions, while capable of yielding greatly enhanced ion exchange properties do not lend themselves to replacing the conventional soda-lime-silica compositions for flat glass because they usually entail sacrificing other desirable properties of the glass and often include costly batch ingredients. For example, relatively high Al.sub.2 O.sub.3 content of some specialized ion exchange glass compositions increases the melting temperature of such glasses and reduces their resistance to chemical attack. Relatively high ZrO.sub.2 contents of some of these modified glass compositions leads to devitrification problems when they are melted and formed into flat glass. Also, ZrO.sub.2 sources can be costly. Thus, it would be desirable if soda-lime-silica glass compositions could have their ion exchange properties enhanced without significantly deviating from the physical properties of commercial flat glass.
Soda-lime-silica flat glass compositions typically fall within the following compositional range on a weight percent basis:
______________________________________ SiO.sub.2 -- 69-75 percent Na.sub.2 O -- 12-16 percent K.sub.2 O -- 0-2 percent CaO -- 8-12 percent MgO -- 2-5 percent Al.sub.2 O.sub.3 -- 0-2 percent ______________________________________
Also included may be up to about 5 percent total of melting and fining aids, such as MnO, TiO.sub.2, ZnO, PbO, SO.sub.3, Sb.sub.2 O.sub.3, As.sub.2 O.sub.3, and colorants such as Fe.sub.2 O.sub.3, NiO, and CoO.